HIV-1,br was isolated from the autopsied brain tissue of a 57-year-old man died of progressive dementing illness. This virus was shown to be HIV-1 by hybridization to HIV-specific DNA probes. The expression of viral proteins as tested by radioimmunoprecipitation assay revealed the presence of HIV-1 specific proteins. HIV-1,br replicated in cultures of CD4+ T-lymphoid cells and induced cytopathic effects in these cells. HIV-1,br also replicated in monocytoid cell lines. The genetic nature of this isolate was determined by molecular cloning and sequencing of the 3'-half of the genome. DNA sequence information established that HIV-l,br is unique HIV-1 isolate. A stretch of approximately 30 bases in the nef gene of HIV-l,br was found duplicated when compared with the other sequenced HIV-1 genomes (Virology, 1989;168:79-89). To determine the functional significance of nef duplication in neuropathogenesis, we have generated primers flanking the duplication. By using these primers we are examining a large number of neural and non-neural isolates of HIV-1 to explore the prevalence of this duplication.